Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As the evolution of batteries continues, and particularly as lithium batteries become more widely accepted for a variety of uses, the need for safe, long lasting high energy batteries becomes more important. There has been considerable interest in recent years in developing high energy density cathode-active materials for use in high energy primary and secondary batteries with alkali-metal anode materials. Several types of cathode materials for the manufacture of thin film lithium and sodium batteries are known in the art. Of considerable interest are cathode materials comprising sulfur-sulfur bonds, wherein high energy capacity and rechargeability are achieved from the electrochemical cleavage (via reduction) and reformation (via oxidation) of these bonds. Sulfur containing cathode materials disclosed for use in lithium and sodium batteries comprise elemental sulfur, organo-sulfur, and carbon-sulfur compositions.
Elemental sulfur is an attractive cathode material in alkali-metal batteries owing to its low equivalent weight, low cost, and low toxicity. Many alkali-metal/sulfur battery cells have been described, as for example, in U.S. Pat. Nos. 3,532,543; 3.907,591; 3,953,231; 4,469,761; 5,532,179; Rauh et al., J. Electrochem. Soc. 1979, 126(4), 523-527; Yamin et cr., J. Electrocheem. Soc. 1988, 135(5), 1045-1048; and Peled, et cil., J. of Power Sources 1989, 26, 269-271.
Many problems with alkali metal/elemental sulfur battery cells have been discovered. One pertains to alkali-metal sulfides formed at the positive electrode on discharge, reacting with elemental sulfur to produce polysulfides that are soluble in the electrolyte causing self-discharge, cell separator clogging, and loss of cell capacity. Another problem is that alkali-metal sulfides once reoxidized on cell charge may lead to the formation of an insulating layer on the positive electrode surface which electrochemically and ionically isolates it from the electroactive elements in the cell, resulting in poor cell reversibility and loss of capacity. The electrically and ionically non-conductive properties of sulfur are a major obstacle that is difficult to overcome in cells comprising elemental sulfur.
Many attempts have been made to improve the electrochemical accessibility of elemental sulfur in cathodes by adsorbing sulfur onto conductive carbons, as extensively reviewed by Kavan et cil., in Electrochimica Acta 1988, 33, 1605-1612, or by complexing elemental sulfur with conjugated (conductive) polymers as described in U.S. Pat. No. 4,664,991 to Pcrichaud et al. Derivatives of polyacetylene and sulfur are described in U.S. Pat. No. 4,739,018 to Armand et al. and extensively reviewed by Novak et al. in Chem. Rev. 1997, 97, 207-281.
Further types of sulfur-containing cathode materials are C.sub.v S materials characterized as surface compounds of carbon with variable compositions, where v is in the range of 4 to 50, as described in U.S. Pat. No. 4,143,214 to Chang et al. U.S. Pat. No. 4,152,491 to Chang et al. relates to the cathode-active materials which include one or more polymer compounds having a plurality of carbon monosulfide units.
U.S. Pat. Nos. 4,833,048 and 4,917,974 to De Jonghe et al. describe a class of cathode materials made of organo-sulfur compounds of the formula (R(S).sub.y).sub.n where y=1 to 6; n=2 to 20, and R is one or more different aliphatic or aromatic organic moieties having one to twenty carbon atoms. One or more oxygen, sulfur, nitrogen or fluorine atoms associated with the chain can also be included when R is an aliphatic chain. The aliphatic chain may be linear or branched, saturated or unsaturated. The aliphatic chain or the aromatic rings may have substituent groups. The preferred form of the cathode material is a simple dimer or (RS).sub.2. When the organic moiety R is a straight or a branched aliphatic chain, such moieties as alkyl, alkenyl, alkynyl, alkoxyalkyl, alkythioalkyl, or aminoalkyl groups and their fluorine derivatives may be included. When the organic moiety comprises an aromatic group, the group may comprise an aryl, arylalkyl or alkylaryl group, including fluorine substituted derivatives, and the ring may also contain one or more nitrogen, sulfur, or oxygen heteroatoms as well.
In the cell developed by De Jonghe et al., the main cathode reaction during discharge of the battery is the breaking and reforming of disulfide bonds. The breaking of a disulfide bond is associated with the formation of an RS.sup.- M.sup.+ ionic complex. The organo-sulfur materials investigated by De Jonghe et al. undergo polymerization (dimerization) and de-polymerization (disulfide cleavage) upon the formation and breaking of the disulfide bonds. The de-polymerization which occurs during the discharging of the cell results in lower molecular weight polymeric and monomeric species, namely soluble anionic organic sulfides, which can dissolve into the electrolyte and cause self-discharge as well as reduced capacity, thereby severely reducing the utility of the organo-sulfur material as a cathode-active material and eventually leading to complete cell failure. The result is an unsatisfactory cycle life having a maximum of about 200 deep discharge-charge cycles, more typically less than 100 cycles as described in J. Electrochem. Soc. 1991, 138. 1891-1895.
U.S. Pat. No. 5,441,831 to Okamoto et al. relates to an electric current producing cell which comprises a cathode containing one or more carbon-sulfur compounds of the formula (CS.sub.x).sub.n, in which x takes values from 1.2 to 2.3 and n is equal to or greater than 2.
Many molecular and polymeric disulfides for use in batteries have been described in a recent review article by Novak et al. (Chem. Rev. 1997, 97, 207-281). The polymeric disulfide polymers generally exhibit slower electrochemical kinetics than the molecular (soluble) organo-disulfide materials. A clear preference for linear disulfide polymers in cathodes was disclosed by Liu et al., (Electrochemical Society Proceedings, 1990, 90-5, 220), wherein crosslinked and ladder type polydisulfide polymers exhibited inferior performance.
Energy storage by the reversible cleavage and reformation of sulfur-sulfur bonds has been reported for organic and polymeric compounds comprising polysulfide, --S.sub.m -- (trisulfide, tetrasulfide, pentasulfide, etc.) moieties. Owing to the presence of multiple linked sulfur atoms in these molecules, they possess higher energy densities than the corresponding molecules containing the disulfide linkage, --S--S--, alone. Skotheim et. el. in U.S. Pat. Nos. 5,601,947, 5,609,702 and 5,529,860, and in U.S. Pat. application Ser. No. 08/602,323 have disclosed that improved capacity and cycleability can be achieved by using cathode active carbon-sulfur cathode polymer materials comprising polysulfide moieties. U.S. Pat. No. 5,529,860 and U.S. Pat. application Ser. No. 08/602,323, now, abandoned describe polyacetylene-co-polysulfur compositions of general formula (C.sub.2 S.sub.x).sub.n, wherein x is greater than 1 to about 100 and n is equal to or greater than 2. U.S. Pat. Nos. 5,601,947 and 5,609,702 describe carbon-sulfur polymer materials of general formula (CS.sub.x).sub.n, wherein x is greater than 2.3 to about 50 and n is equal to or greater than 2, which in their oxidized state, comprise a polysulfide moiety of the formula, --S.sub.m --, wherein m is an integer equal to or greater than 3. The polyacetylene-co-polysulfur and the carbon-sulfur polymer materials may comprise covalently bound polysulfide, --S.sub.m --, sidechains on the polymer backbone chain, as well as polysulfide, --S.sub.m --, main chain linkages in the polymer backbone.
Naoi et al. (Electrochemical Society Proceedings, 1996, 14, 131-138, Abstract No. 142, Joint International Meeting of Electrochemical Society and International Society of Electrochemistry, Paris, Aug. 21-Sep. 5, 1997, J Electrochem. Soc. 1997, 144(6), L1170-172 and Japanese Patent Application No. 09-139213, published May 27, 1997) have recently reported higher discharge capacities for the molecular trisulfide and tetrasulfide dimers and polymers compared to the corresponding disulfide dimers and polymers. However, they also observed that charge retention (capacity) on electrochemical cycling of the trisulfide and tetrasulfide compounds was less than that of the disulfide dimers.
Despite the various approaches proposed for the fabrication of high energy density rechargeable cells containing elemental sulfur, organo-sulfur and carbon-sulfur cathode materials, or derivatives and combinations thereof, there remains a need for materials that improve the utilization of electroactive cathode materials and the cell efficiencies and provide rechargeable cells with high sustainable capacities over many cycles.
It is therefore an object of the present invention to provide composite cathodes containing high loadings of electroactive sulfur-containing cathode material that exhibit a high utilization of the available electrochemical energy and retain this energy capacity without significant loss over many charge-discharge cycles.
It is another object of the present invention to provide high sulfur content polymers and copolymers useful as cathode materials with high surface area and open structures that exhibit high charge and discharge rates, and to provide processes for making such high sulfur content polymers and copolymers.
It is a further object of this invention to provide methods for fabricating cathode elements comprising the high sulfur content polymers and copolymers of the present invention.
It is yet a further objective of this invention to provide energy-storing rechargeable battery cells which incorporate such composite cathodes, and which exhibit much improved self-discharge characteristics, long shelf life, improved capacity, and high manufacturing reliability.